1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus such as a copying apparatus or a printer for developing an electrostatic image.
2. Related Background Art
FIG. 15 shows an example of a developing unit used in an electrophotographic apparatus.
In FIG. 15, a developing device 109 arranged to face a photoconductive drum 104 has a developing container 108, a developing sleeve 103 as developing agent carrying means, a developing agent return member 101 for defining a gathering part 105 for the developing agent and a blade 102 as a height defining member for the developing agent. An interior of the developing unit 109 is partitioned to a developing chamber (first chamber) 113 and an agitation chamber (second chamber) 114 by a wall 106 extending vertically, and the top of the wall 106 is open. Two-component developing agent including non-magnetic toner and magnetic carrier is accommodated in the developing chamber 113 and the agitation chamber 114, and the extra developing agent in the developing chamber 113 is collected into the agitation chamber 114.
First and second agitation screws 111 and 112 are arranged in the developing chamber 113 and the agitation chamber 114, respectively.
In the developing chamber 113 of the developing device 109, an opening is formed at a position corresponding to a developing area facing the photoconductive drum 104, and the developing sleeve 103 is rotatably arranged so as to be partially exposed to the opening. The developing sleeve 103 is made of a non-magnetic material and rotated in a direction of an arrow in FIG. 15 during a developing operation. A magnet (magnet roller) 110 which is a magnetic field generation means is fixed in the developing sleeve 103. The developing sleeve 103 carries the two-component developing agent having the layer height regulated by the blade 102 and supplies the developing agent to the photoconductive drum 104 in the developing area facing the photoconductive drum 104 to develop a latent image. In order to improve the developing efficiency, a developing bias voltage having an AC voltage superimposed on a DC voltage is applied from a power supply 115 to the developing sleeve 103.
According to the construction mentioned above, the developing device 109 holds the developing agent supplied to the surface of the developing sleeve 103 by the agitation screws 111 and 112 in a state of magnetic brush by the magnetic force of the magent roller 110, and carries it to the area facing the photoconductive drum 104 (developing area) by the rotation of the developing sleeve 103, and cuts the magnetic brush by the developing agent return member 101 and the blade 102 to properly maintain the amount of developing agent carried into the developing area.
More particularly, the magnet roller 110 of the conventional developing device is of 5-pole construction, and the developing agent agitated by the developing chamber agitation screw 111 is bound by the magnetic force of a carrying magnetic pole (pumping pole) S2 for pumping and it is carried to the developing agent gathering part 105 by the rotation of the developing sleeve 103. The amount of the developing agent is regulated by the developing agent return member 101 and sufficiently bound by a carrying magnetic pole (cut pole) N2 having a predetermined or higher magnetic flux density to bind stable developing agent, and carried while forming the magnetic brush. The magnetic brush is then cut by the blade or the height regulation member 102 to maintain the proper amount of developing agent, and the developing agent is carried by a carrying magnetic pole S1. The bias voltage having the DC and/or AC voltage superimposed is applied to the developing sleeve 103 through the bias power supply 115 provided in the image forming apparatus at a developing pole N1, and the toner on the developing sleeve 103 is moved toward the electrostatic latent image, which is visualized as a toner image.
In such a developing device, a portion of the toner or the developing agent scatters as the developing sleeve 103 is rotated to contaminate the image or deposit on other equipments nearby, which causes various troubles. Various approaches have been adapted in the past to prevent such scattering.
In the example of FIG. 15, a magnetic plate 123 is arranged in the vicinity of a carrying magnetic pole (take-in pole) S3 to form a magnetic curtain by the developing agent to prevent the scattering. In Japanese Laid-Open Patent Application No. 4-6949, the conductive member which is close to the photoconductive drum is electrically insulated and arranged at a developing agent supply port of the developing device to form an electric field curtain to prevent the scattering of the toner. In Japanese Laid-Open Patent Application No. 4-51026, an electrode member is provided at a portion of the developing sleeve which is close to the photoconductive drum to generate a uni-directional electric field between the electrode member and the developing sleeve.
However, in the magnetic curtain by the magnetic plate, a sufficient magnetic curtain for preventing the scattering may not be formed when the magnetic flux density of the carrying magnetic pole S3 is not sufficiently large, or it is not effective for the scattering generated in the developing pole N1 from the S3 pole.
In the method disclosed in Japanese Laid-Open Patent Application No. 4-6949, the suppression of the scattering to the longitudinally opposite ends of the developing device, that is, to the front end and the rear end, is not effective.
In the method disclosed in Japanese Laid-Open Patent Application No. 4-51026, the electric field established by the electrode member downstream of the developing area as viewed in the direction of rotation of the sleeve may be directed to the developing area and the scattering near the take-in port of the developing agent cannot be perfectly suppressed.
Japanese Laid-Open Patent Application No. 58-46365, Japanese Laid-Open Patent Application No. 60-95574 and Japanese Laid-Open Patent Application No. 60-125863 disclose to apply a voltage of the same polarity as that of the toner to the electrode, but the effect of suppressing the scattering is not sufficient.